The present application relates to a process for providing semiconducting devices, in particular transistors and solar cells and to the devices obtainable by such a process.
Semiconducting devices have a wide variety of applications. For example thin film transistors, TFTs incorporating amorphous silicon or polycrystalline silicon are employed as current switching devices in applications such as the active matrix of liquid crystal displays.
A thin-film transistor 40 (FIG. 3) is in its most simple form a three-terminal device, i.e. it has three separate metal contacts.
An example of the TFT composition is as follows: a flat wafer 42 made of highly doped crystalline silicon is thermally oxidized on one face. The oxide layer 44 is a non-conducting or insulating layer. Alternative materials, including but not limited to silicon nitride or silicon oxy-nitride, can also function as the insulator. On the opposing face of the wafer a metal contact is applied, the gate contact 46. Then a semiconducting material, e.g. poly-crystalline silicon or silicon or hydrogenated amorphous silicon (xcex1-Si:H), is applied to the side of the wafer where the insulator is present 48. On top of this semiconducting layer 48 a second silicon layer 50, which contains dopant atoms such as phosphorus or boron, is applied. Then on top of this, two metal contacts for the source 52 and drain 54 are applied adjacent to each other with a set and fixed distance between them. The highly conductive lastly applied silicon layer is removed from between the source and drain contacts, hence creating a semi-insulating path between these contacts.
When a voltage is applied to the gate contact a highly conductive sublayer is created in the semiconductor, parallel to and spatially adjacent to the interface of the insulator and semiconductor. This layer is called the channel of the TFT and is typically 50 xc3x85ngstrom to 200 xc3x85ngstrom thick. The effect of enhanced conductivity upon application of a voltage to the gate electrode is the field effect. When an electric field is present between the source and drain, current conduction through the TFT takes place from the source to the interface, then laterally along the interface if a voltage is applied to the gate, and out perpendicular to the said interface through the drain.
In thin-film transistors based on hydrogenated amorphous silicon, a shift to higher voltages at the onset of current conduction (the threshold voltage) is commonly observed upon prolonged application of a positive voltage to the gate terminal of the device. Also a deterioration of the subthreshold slope is observed. When a negative voltage is applied, a shift of the threshold voltage is also observed.
Conventional techniques for depositing a semiconducting layer onto a substrate include the so-called radiofrequency glow discharge technique at 13.56 MHz (a form of plasma enhanced chemical vapour deposition) and also at higher excitation frequencies in the range of 13.56 MHz to 150 MHz.
A problem of xcex1-Si:H in electronic devices such as solar cells, diodes, and TFTs, is its metastable behavior upon application of a continuous voltage to one of the electrodes or to the gate electrode of TFTs and upon illumination with light.
For example in flat screen televisions comprising an active matrix, where the screens consist of a number of pixels, each pixel has a transistor which can be switched on and off to allow light to pass through, whereby the clarity of the image can be controlled. Transistors incorporating xcex1-Si:H as the semiconducting material exhibit an increase in gate threshold voltage to achieve the same current, sometimes after only a half hour.
Furthermore, light illumination of a solar cell causes the creation of electronic defects which results in deterioration of the device performance.
These effects are reversible, for instance when the device is heated to about 150xc2x0 C., the original device characteristics can be re-obtained. However this heat treatment is neither a practical nor economical solution for the many applications comprising these devices.
In thin film transistors incorporating an xcex1-Si:H semiconducting layer, the creation of electronic defects is caused by a continuous voltage applied to the gate contact. A shift in the gate voltage at the onset of current conduction, the so-called threshold voltage, is generally observed, even after only a few minutes of gate voltage application. This means that after several minutes of operation, a higher voltage is required to turn on the transistor, which results in the unwanted effect of drift in the operational characteristics. This effect is illustrated in FIGS. 4 and 7. In FIG. 4 the characteristics are shown for a TFT with the xcex1-Si:H deposited at 50 MHz, in FIG. 7 those for a 13.56 MHz TFT. The curves are for xe2x80x9cas depositedxe2x80x9d, after 1.5 h stress, and after 2.5 h stress, respectively.
An object of the present invention is to provide improved semiconductor devices.
According to a first aspect the present invention provides a process for providing a semiconducting device comprising the steps of:
depositing a semiconducting layer onto a substrate by means of heating a gas to a predetermined, dissociation temperature so that the gas dissociates whereby fractions thereof condense on the substrate to build up a semiconducting layer.
The inventors have found that depositing a semiconducting layer onto a substrate in this manner yields a semiconducting device which exhibits substantially no shift in threshold voltage upon gate voltage application.
According to a further aspect of the present invention there is provided a device, in particular being a transistor, said device having a substantially consistent gate voltage and a saturation mobility xcexc, in the range of about 0.001 to about 100, for example about 0.001 to about 10 and most preferably from about 0.1 to about 1.00 cm2/V.s.
According to yet another aspect of the present invention, there is provided a device comprising a substantially exclusive polycrystalline Si:H or a polycrystalline and amorphous Si:H layer, said device having a substantially consistent gate voltage and a saturation mobility lying in the range of about 0.001-1000, for example 0.001 to 500 cm2/V.s.
Although exhibiting a similar saturation mobility to TFT""s according to the present invention, conventional TFT""s still suffer from dramatically increasing threshold voltages during their working lives.
According to another aspect of the present invention, there is provided a semiconducting device obtainable according to the above process.
According to a further aspect of the present invention there is provided a vacuum chamber for carrying out the above process.